1. Field of Invention
The present invention relates to an improved way of processing analog scan data signals generated from laser scanning bar code symbol reading systems, and more particularly, to provide an improved way of and means for processing analog scan data signals, including first and second derivative signals derived therefrom, so that the effects of thermal noise and substrate/paper noise alike are minimized in diverse laser scanning environments including, multiple focal zone scanning systems and large depth-of-field scanning systems alike.
2. Brief Description of State of the Art
Code symbol scanners are widely used in diverse environments for purposes of object identification, data-entry and the like.
During operation of such machines, a focused light beam is produced from a light source such as a visible laser diode (VLD), and repeatedly scanned across the elements of the code symbol attached, printed or otherwise fixed to the object to be identified. In the case of bar code scanning applications, the elements of the code symbol consists of a series of bar and space elements of varying width. For discrimination purposes, the bars and spaces have different light reflectivity (e.g. the spaces are highly light-reflective while the bars are highly light-absorptive). As the laser beam is scanned across the bar code elements, the bar elements absorb a substantial portion of the laser beam power, whereas the space elements reflect a substantial portion thereof. As a result of this scanning process, the intensity of the laser beam is modulated in accordance with the information structure encoded within the scanned bar code symbol. As the laser beam is scanned across the bar code symbol, a portion of the reflected light beam is collected by optics within the scanner. The collected light signal is subsequently focused upon a photo-detector within the scanner which generates an analog electrical output signal which can be decomposed into a number of signal components, namely: a digital scan data signal having first and second signal levels, corresponding to the bars and spaces within the scanned code symbol; ambient-light noise produced as a result of ambient light collected by the light collection optics of the system; thermal noise produced as a result of thermal activity within the signal detecting and processing circuitry; and “paper” or substrate noise produced as a result of the microstructure of the substrate in relation to the cross-sectional dimensions of the focused laser scanning beam. The analog scan data signal has positive-going transitions and negative-going transitions which signify transitions between bars and spaces in the scanned bar code symbol. However, a result of such noise components, the transitions from the first signal level to the second signal level and vice versa are not perfectly sharp, or instantaneous, as in the underlying digital scan data signal. Consequently, it is difficult to determine the exact instant that each binary signal level transition occurs in the detected analog scan data signal.
It is well known that the ability of a scanner to accurately scan a bar code symbol and accurately produce digital scan data signals representative of a scanned bar code symbol in noisy environments depends on the depth of modulation of the laser scanning beam. The depth of modulation of the laser scanning beam, in turn, depends on several important factors, namely: the ratio of the laser beam cross-sectional dimensions at the scanning plane to the width of the minimal bar code element in the bar code symbol being scanned, and (ii) the signal to noise ratio (SNR) in the scan data signal processor at the stage where binary level (1-bit) analog to digital (A/D) signal conversion occurs.
As a practical matter, it is not possible in most instances to produce analog scan data signals with precisely-defined signal level transitions. Therefore, the analog scan data signal must be further processed to precisely determine the point at which the signal level transitions occur.
Hitherto, various circuits have been developed for carrying out such scan data signal processing operations. Typically, signal processing circuits capable of performing such operations include filters for removing unwanted noise components, and signal thresholding devices for rejecting signal components which do not exceed a predetermined signal level.
One very popular approach for converting analog scan data signals into digital scan data signals is disclosed in U.S. Pat. No. 4,000,397, incorporated herein by reference in its entirety. In this US Letters Patent, a method and apparatus are disclosed for precisely detecting the time of transitions between the binary levels of encoded analog scan data signals produced from various types of scanning devices. According to this prior art method, the first signal processing step involves double-differentiating the analog scan data input signal Sanalog to produce a second derivative signal S″analog. Then the zero-crossings of the second derivative signal are detected, during selected gating periods, to signify the precise time at which each transition between binary signal levels occurs. As taught in this US patent, the selected gating periods are determined using a first derivative signal S′analog formed by differentiating the input scan data signal Sanalog. Whenever the first derivative signal S′analog exceeds a threshold level using peak-detection, the gating period is present and the second derivative signal S″analog is detected for zero-crossings. At each time instant when a second-derivative zero-crossing is detected, a binary signal level is produced at the output of the signal processor. The binary output signal level is a logical “1” when the detected signal level falls below the threshold at the gating interval, and a logical “0” when the detected signal level falls above the threshold at the gating interval. The output digital signal Sdigital produced by this signal processing technique corresponds to the digital scan data signal component contributing to the underlying structure of the analog scan data input signal Sanalog.
While the above-described signal processing technique generates a simple way of generating a digital scan data signal from a corresponding analog scan data signal, this method has a number of shortcomings and drawbacks.
In particular, thermal as well as “paper” or substrate noise imparted to the analog scan data input signal Sanalog tends to generate zero-crossings in the second-derivative signal S″analog in much the same manner as does binary signal level transitions encoded in the input analog scan data signal Sanalog. Consequently, the gating signal mechanism disclosed in U.S. Pat. No. 4,000,397 allows “false” second-derivative zero-crossing signals to be passed onto the second-derivative zero-crossing detector thereof, thereby producing erroneous binary signal levels at the output stage of this prior art signal processor. In turn, error-ridden digital data scan data signals are transmitted to the digital scan data signal processor of the bar code scanner for conversion into digital words representative of the length of the binary signal levels in the digital scan data signal. This can result in significant errors during bar code symbol decoding operations, causing objects to be incorrectly identified and/or erroneous data to be entered into a host system.
Also, when scanning bar code symbols within large scanning fields volumes having multiple focal zones, as taught in co-applicant'PCT International Patent Publication No. WO 97/22945 published on Jun. 26, 1997, Applicants have observed that the effects of paper/substrate noise are greatly amplified when scanning bar code symbols in the near focal zone(s) of the system, thereby causing a significant decrease in overall system performance. In the far out focal zones of the scanning system, Applicants have observed that laser beam spot speed is greatest and the analog scan data signals produced therefrom are time-compressed relative to analog scan data signals produced from bar code symbols scanned in focal zones closer to the scanning system. Thus, in such prior art laser scanning systems, Applicants have provided, between the first and second differentiator stages of the scan data signal processor thereof, a low-pass filter (LHF) having cutoff frequency which passes (to the second differentiator stage) the spectral components of analog scan data signals produced when scanning bar code elements at the focal zone furthest out from the scanning system. While this technique has allowed prior art scanning systems to scan bar codes in the far focal zones of the system, it has in no way addressed or provided a solution to the problem of increased paper/substrate noise encountered when scanning bar code symbols in the near focal zones of such laser scanning systems.
Thus, there is a great need in the art for an improved analog scan data signal processing device and method which enables precise detection of signal level transitions in analog scan data signals produced when scanning bar code symbols using a multi-focal zone laser scanning system, while mitigating the effects of thermal and paper noise encountered when scanning bar code symbols in both the near and far focal zones thereof.